Cell-cell and cell-extracellular matrix (ECM) interactions play a crucial role in the normal development of the nervous system, as well as in its response to injury. Despite our detailed knowledge of the sequence of these events, little is known about the mechanisms that control them. We propose to examine one component of the ECM, thrombospondin (TSP), which we have shown to be highly developmentally regulated during the histogenesis of the nervous system (NS). We have previously demonstrated a critical role in TSP in the adhesion, migration, process outgrowth of neuronal cells, and recently, in glial maturation. These multiple functions of TSP have been attributed to a single molecule, but new exciting data indicate that there ar multiple TSPs encoded by different genes. TSP1 and TSP2. Despite the presence of TSP3 in the developing CNS, our investigations indicate that neurons fail to attach to TSP3, while glial cells differentiate on TSP3 coated substrates. After critical periods of cell migration and process outgrowth, TSP levels drop drastically in the CNS; although TSP expression remains high in the PNS into adulthood. Consistent with this observation, TSP levels increase significantly following peripheral nerve injury in regions of reunite - Schwann cell contract. Similarily, neuron:glial interactions appear to be crucial in the formation of the cerebellar cortex where neuronally derived TSP may mediate astroglial maturation. In the current investigation, we will determine the detailed pattern of expression of TSP1,2,3 in the development of peripheral nerve and cerebellar cortex, and examine the cell biological response of neurons and glial cells to the TSPs. A series of experiments will be carried out to determine th role of TSP1, 2, 3 in regeneration and in cerebellar development using available null mutations and following the production (by us) of a TSP3 null mouse. The nature and extent of recovery following sciatic nerve injury will be determined in adult control and in TSP null mice. Neurons and glial cells from these mice will also be specifically recombined to conclusively determine the role of individual TSPs in neuron; glial interactions in both CNS and PNS. Given the ability of TSP1 & 2 to stimulate neurite extension and cell migration, and TSP3 to promote glial cell maturation, this integrative approach should elucidate the unique roles of the TSPs in basic cell biological questions of process outgrowth, neuronal;glial interactions, and will provide valuable new data on the response of the nervous system to injury.